1. Field of the Invention
The present invention relates to an arrangement for measurement demodulation and modulation error measurement of a digitally modulated receive signal.
2. Description of the Background Art
Measuring arrangements of this type are known (ETSI Tdoc SMG2 829/99, Meyr, Moeneclaey, Fechtel: “Digital Communication Receivers”, WILEY INC, New York, 1997). They are used for modulation error measurement on digitally modulated receive signals. For data transmission in modern digital transmission technology, so-called digital modulation modes, which are known in many different variants, are used. The most frequently used modulation modes are the so-called PSK, QAM, MSK or FSK modulation methods. For data transmission, special transmit and receive filters are required at the transmitting and receiving ends, respectively, in order to achieve time intervals for the demodulation of the data, that are free of inter-symbol interference. For measurement purposes, special weighting filters have to be used in the receive path, instead of the receive filter.
FIG. 1 shows a known filter arrangement that is suitable for this purpose. The PSK modulated measuring signal, for example, passes after frequency conversion, not shown, A/D conversion and, if required, digital mixing, to the input of the measuring arrangement as a complex baseband signal. Receive filtering in a receive filter 1 (matched filter) initially takes place at this point, and in the following demodulator 2 signal errors such as mean frequency errors, initial phase errors, mean timing errors and the like are detected and eliminated. Furthermore, a symbol decision stage is provided in the demodulator 2, which generates the symbol samples of an ideal, reconstructed transmit signal from the error-free measuring signal, for example by quantisation of the IQ data, the phase or absolute value. The signal is then filtered by means of a reference filter 13. The reference filter 13 has the characteristic: reference filter=TX filter*weighting filter.
(Here the symbol “*” is used as a convolution operator and signifies convolution of the filter pulse responses in the time domain; both in the time domain and in the frequency domain the filter design itself can be achieved analytically and with approximation methods).
In this case the TX filter is the pulse-shaping filter used at the transmitting end of the respective transmission system. A weighting filter 11 is a filter that is specified according to the weighting standard. The input signal to be weighted in the weighting filter 11 is first delayed in a memory 9 and error-corrected in an error-correction stage 10 that is connected to the demodulator 2, and is then fed to the weighting filter 11. This weighting filter 11 is designed in accordance with the desired weighting function, for example in accordance with the ETSI specification. The ideal signal of the reference filter 13 and the weighted receive signal of the weighting filter 11 is then passed on to a following evaluation circuit 4 for further error detection, and finally to a display circuit 5 in which, in addition to the detected numerical modulation errors, measuring or reference signals as well as error signals derived from them, are numerically or graphically displayed. For further error detection in the evaluation circuit 4, for example, by comparing the two signals, further modulation errors, for example, error vector magnitude, magnitude error, phase error, respectively, are determined symbol-by-symbol or over a specific measurement period.
In the known arrangement, the original input signal has to be temporarily stored in a memory for later weighting filtering, and additional arithmetic operations are necessary for error correction of the original input signal prior to its weighting.